1. Field of the Invention
This invention relates to a novel additive for aqueous drilling fluids and a method for drilling wells in subsurface formations utilizing drilling fluids containing said additive, and particularly to an additive for reducing the gel strengths and yield points of high pH, fresh water drilling fluids.
2. Prior Art
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clayey solids used in the drilling of wells in the earth, such as are drilled for the purpose of tapping subterranean deposits of petroleum, gas, and other fluid materials. Drilling fluids have a number of functions, the most important of which are removing formation cuttings from the well, sealing off permeable formations of gas, oil or water which may be encountered at various levels as the well is drilled into the subterranean formation, cooling and lubricating the drilling tool and drill pipe which carries the tool, and holding the cuttings in suspension in the event of shut down in the drilling and pumping of the drilling fluid.
An ideal drilling fluid is a thixotropic fluid, i.e., a fluid whose viscosity decreases as the degree of agitation or shear rate increases. In the drilling of wells utilizing drilling fluids, agitation or shearing caused by circulation is halted, however, the fluid gels or forms a gel structure which will support the drilled cuttings to prevent them from falling down into the bottom of the hole. The rate of gel formation must be such as to permit the cuttings to fall only a short distance before the gel structure is sufficiently strong to support them. It is important to maintain the degree of gelation and the rate of gelation within narrow limits, since excessive gel formation would be detrimental to the resumption of drilling operations, and will also tend to entrain gas in the drilling fluid. Entrainment of gas in the drilling fluid leads to a substantial reduction in the density of the drilling fluid, which can be detrimental to continued safe drilling of the well, especially if high pressure formations are to be encountered. If the gelation rate and/or gel strength are below the desired range, formation cutting and other solid materials such as weighting material will fall to the bottom of the hole which results in sticking of the drill pipe.
Fresh water drilling fluids, e.g. aqueous drilling fluids containing little or no lime or other water soluble calcium salts, are widely employed in drilling operations. Usually some substance must be similarly employed in fresh water drilling fluids to control gelation rate and degree of gelation or gel strength within acceptable operating limits. Agents commonly employed for this purpose include lignosulfonates, and dextrin. Most commercial thinners are not always entirely satisfactory for a variety of reasons, including sensitivity thereof to high temperatures, contamination with extraneous substances sometimes encountered in drilling and high material costs. Also, many commercially available dispersants are ineffective in the presence of calcium and/or sodium chloride. If the fluid is converted to some specialized fluid at a later time, which involves adding calcium salts or sodium chloride the effectiveness of the fresh water fluid will be lost and so more dispersant will be requird than would be the case if the fresh water fluid had not lost its effectiveness.
Thus it can be appreciated that there is a substantial need for having additional dispersants for fresh water drilling fluids to meet varying conditions encountered in the field and especially which will be effective when the drilling fluid is converted to a chemicalized system.